Thanks so much for the answers to my questions.
The fact that he's five years of age and behaving as you describe says to me that changing or modifying this behavior is going to take some work; there aren't going to be any simple or quick fixes, I'm afraid. But this is often the case when it comes to inappropriate behavior in our pets.
However, if both your brother and sister in law are motivated to change this behavior, then I do think there are some things they can try. I don't think euthanasia is an option that I would consider at this point.
In order to try and change this behavior, a more positive association with strangers/men needs to be created.
First, he needs to learn basic obedience commands such as sit/stay. He needs to learn them so well that the instant the command is given, he obeys it.
I would use really yummy foods such as steak or meat or small amounts of hot dogs...something that he really likes a lot and are only used during the training sessions.
Enlist the aid of a friend, or neighbor...someone he doesn't know. Then he needs to sit/stay when they ring the bell or knock on the door; he is still receiving praise and food during this exercise. Once he doesn't react to the knock or the ringing, then the door is opened, and the visitor just stands in the door with no attempt to come inside.
Eventually, the stranger comes into the house but doesn't talk to him, doesn't make eye contact with him ..basically ignores him. All the while either your brother or sister in law have control over him with him sitting/staying and being fed his treats. Once he's comfortable with the distance the man/stranger is at, then he moves a little closer.
I think you see the pattern here. Slow, baby steps until he is comfortable with each phase before they move on to the next one. They can't rush the process. Everything is very low key; they are speaking in a firm but calm voice but they are making the training sessions fun, too. Training sessions should last about 10-15 minutes at a time with multiple sessions during the day, if possible.
At some point, once he seems really comfortable with a stranger in the room, they can feed him too but only if he is in the sit/stay position.
Obviously, this will take a great deal of time and patience but I do believe that you can change his reactions/behaviors unless they are so ingrained at this point that consultation with a behavioral specialist may be needed. The advantage of a professional veterinarian trained in this discipline is that they can also prescribe medication if necessary after evaluation of the situation.
If you've been around dogs for a while, you know that some dogs take a dislike to someone for whatever reason. This may still happen with him; then he should be taken to a different room and not forced to deal with that person. But this should not happen very often once he loses the fear of strangers.
Additional helpful tips:
He must earn everything...the basic principal of "nothing in life is free". He sits/stays before he is fed, he sits before he goes through a door, he sits before his leash is put on for his walk, etc. Initially, I would reward him with a treat. everytime he obeys, then I would intermittently reward him (sometimes he gets a treat but other times he doesn't) and eventually he is rarely rewarded with food.
Everyone that he comes in contact with should make him obey commands before they pet him, before they give him treats, etc.
I can't emphasize enough how difficult I know this can be and how time consuming but positive reinforcement techniques are more likely to work than things like shock collars. And these techniques always take a great deal of time and patience and repetitive actions/rewards to be effective.
And, I would strongly encourage the advice of a professional behavioral specialist before I would consider euthanasia.
I hope this helps and I wish your family the best with this situation. Deb